Rare earth element

The rare earth elements or rare earth metals are a relatively abundant group of 17 chemical elements (see Periodic Table) composed of scandium, yttrium, and the lanthanides. The lanthanide promethium, which does not occur naturally on Earth, is generally not considered a rare earth element.

The rare earth elements are named after their oxides, the rare earths. At the time of their discovery, these were difficult to extract from the rocks that contained them and thought to be scarce. Sometimes rare earth elements are loosely referred to as 'rare earths'.

The elemental forms of rare earths are iron gray to silvery lustrous metals that are typically soft, malleable, and ductile and usually reactive, especially at elevated temperatures or when finely divided. All rare earth elements dissolve in acid to form triply charged ions in solution, and +3 is the default oxidation state.

The rare earths' unique properties are used in a wide variety of applications. For example, their ions are widely used as dopants in active laser media such as Nd3+:YAG (neodymium-doped yttrium aluminum garnet) and Erbium-doped fiber amplifiers. Half-full f shells in these elements can be used to produce exceedingly strong permanent magnets, with Samarium-Cobalt holding the record for highest Curie temperature and Neodymium-Iron-Boron having the highest known permanent magnetization of any material.

The principal economic sources of rare earths are the minerals bastnasite, monazite, and loparite and the lateritic ion-adsorption clays. The moniker "rare earth" can be misleading – the elements range in crustal abundance from cerium, the 25th most abundant element of the 78 common elements in the Earth's crust at 60 parts per million (more common than lead), to thulium and lutetium, the least abundant rare-earth elements at about 0.5 parts per million.